Irrigation and frost preventation method and apparatus

ABSTRACT

A method of and system for protecting plantings such as trees, plants, and the like, against frost damage by conducting a heated liquid into heat transfer relation with thermal radiators located adjacent the base of the plantings to heat the radiators and thereby create convection currents which circulate about the plantings to lower the minimum air temperature which the plantings can withstand without permanent frost damage and raise the air temperature about the plantings by the conjoint action of heating the air and circulating the relatively warm air from the air space above the plantings to the air space about the plantings. A combined irrigation and frost protection system which is selectively operable in an irrigation mode wherein an irrigation liquid is supplied to the plantings through dispensers, preferably drip dispensers, located adjacent the base of the plantings, and a frost protection mode wherein the irrigation liquid is heated and utilized to heat thermal radiators mounted on the dispensers for creating convection air currents about the plantings. The combined system may also be operable in a fertilizing mode wherein a fertilizing agent is introduced into the irrigation liquid.

United States Patent [1 1 Mclsaac [451 Sept. 4, 1973 IRRIGATION ANDFROST PREVENTATION METHOD AND APPARATUS Donald N. Mclsaac, 639 AdelaideDr., Santa Monica, Calif.

[76] Inventor:

[57] ABSTRACT A method of and system for protecting plantings such astrees, plants, and the like, against frost damage by conducting a heatedliquid into heat transfer relation with thermal radiators locatedadjacent the base of the plantings to heat the radiators and therebycreate convection currents which circulate about the plantings to lowerthe minimum air temperature which the plantings can withstand withoutpermanent frost damage and raise the air temperature about the plantingsby the conjoint action of heating the air and circulating the relativelywarm air from the air space above the plantings to the air space aboutthe plantings.

A combined irrigation and frost protection system which is selectivelyoperable in an irrigation mode wherein an irrigation liquid is suppliedto the plantings through dispensers, preferably drip dispensers, locatedadjacent the base of the plantings, and a frost protection mode whereinthe irrigation liquid is heated and utilized to heat thermal radiatorsmounted on the dispensers for creating convection air currents about theplantings. The combined system may also be operable in a fertilizingmode wherein a fertilizing agent is introduced into the irrigationliquid.

11 Claims, 13 Drawing Figures [52] 11.5. C1. 47/2 [51] A01g 13/00 [58]Field of Search 47/2; 239/542; 128/595; 137/334 [56] References Cited 1UNITED STATES PATENTS 1,758,941 5/1930 Gibson 47/2 X 3,016,202 1/1962Zalenski.. 239/542 2,154,002 4/1939 Kerrick 47/2 1,846,395 2/1932Huffaker 47/2 2,155,184 4/1939 Fujiusa 47/2 3,604,728 9/1971 Symcha..239/542 X 1,021,691 3/1912 McAdie 47/2 X 661,898 11/1900 Tucker... 47/22,324,234 7/1943 Peters 239/542 X Primary Examiner-Robert E. PulfreyAssistant ExaminerE. M. Coven Att0rneyGerald Singer PATENTEDSEP 4 msSHEH 1 0F 2 Water 24" Source 44 48 Filter "0 Heater Fig.

Reservoir e b m Regu lo'ior 4o Pressure b b b hdA h Donald N. McIsouc,

ENTOR. Y.

AGENT.

PATENTEB E? 4 Slim 8 BF 2 Fig. IO.

Fig. 8.

Fig. 9.

Donald N. Mclsouc,

AGENT.

i i iiiiiiiiii BACKGROUND OF THE INVENTION 1. Field of the Invention:

This invention relates generally to the horticultural field and moreparticularly to a novel method of and system for irrigating, fertilizingand/or protecting against frost damage plantings such as trees, plantsand the like.

2. Discussion of the Prior Art:

As will become readily apparent from the ensuing description, thepresent invention may be utilized to irrigate, fertilize and/or protectagainst frost damage a variety of trees, plants and the like. In thisregard, it is important to note at the outset that in the presentdisclosure, the term planting" is used in a generic sense to include alltypes of trees, plants and the like which require irrigation and frostprotection. The term grove" is used in a generic sense to include allarrangements of such plantings which are grown in large numbers forcommercial purposes. However, the invention is particularly concernedwith and will be described in connection with the irrigation and frostprotection of crop-bearing trees, such as fruit trees, nut trees, andthe like.

It is well known by those versed in the art to which the presentinvention pertains that most if not all cropbearing trees are prone tofrost damage when the ambient air temperature drops close to freezing.Moreover, the range between the lowest temperature which such trees canwithstand without irreparable or permanent frost damage and thetemperature which the trees can withstand without any significant damageis extremely narrow. For example, in the case of certain trees, thistemperature range is on the order of 4. Thus, an ambient air temperatureof 28 or lower will cause permanent damage to such trees, while atemperature of 31 or higher will cause no significant damage. At airtemperatures within this range, the trees may sustain some degree offrost damage but generally may be saved.

Various methods have been devised to protect large groves of such treesagainst frost damage. One of these methods involves the use of so-calledsmudge pots to heat the air and create convection air currents in thetree grove. Another method involves the use of large motor driven fansto induce air circulation in the grove. Such air circulation has thetwo-fold effect of lowering the minimum air temperature which the treescan withstand without permanent frost damage and raising the airtemperature about the trees.

In connection with the two-fold effect of air circulation, it should benoted that while such air circulation is definitely known to lower theminimum air temperature which trees and plants can withstand withoutpermanent damage, the exact reasons for this phenomenon are notunderstood. On the other hand, the reason why air circulation raises theair temperature is known and is quite simple. Thus, colder air tends todescend and warmer air tends to raise, and air circulation merelycirculates the relatively warm air from the air space above the trees tothe air space about the trees.

While both the smudge pot and air circulation methods of frostprotection are in current widespread use, they suffer from certainserious inherent disadvantages which this invention overcomes. Bothsystems, for example, are quite costly to install and maintain. Further,the smudge pot method produces air contamination and requires a largenumber of pots which must be either individually fueled and ignited orprovided with a costly central fueling and automatic ignition system.

Air circulating fans, on the other hand, are noisy and present periodicservicing, maintenance, and repair problems.

SUMMARY OF THE INVENTION One important aspect of the present inventionis concerned with a novel method of and system for inducing aircirculation in a grove of crop-bearing trees or other plantings toprotect them against frost damage without the above noted and otherdisadvantages of the existing smudge pot and fan systems. According tothis inventive aspect, one or more thermal radiators are placed adjacentthe base of each planting, and a heated liquid, such as water, isconducted into heat transfer relation with the radiatiors to heat thelatter. The heated radiators produce convection air currents about theplantings as well as slight heating of the air. The convection aircurrents protect the plantings against frost damage by the two-foldaction explained earlier in connection with the existing fan systems.Thus, the convection air currents lower the minimum air temperaturewhich the plantings can withstand without permanent frost damage andcause circulation of the relatively warm air from the air space abovethe plantings to the air space about the plantings.

According to another important aspect of the invention, the frostprotection system is combined with an irrigation system of the kindhaving one or more irrigation liquid dispensers at the base of eachplanting to provide a combined irrigation and frost protection system.The combined system is equipped with means for selectively heating theirrigation liquid and a thermal radiator on each liquid dispenser. Thesystem is selectively operable in an irrigation mode wherein unheatedirrigation liquid is supplied to the dispensers to irrigate theplantings and a frost protection mode wherein the liquid is heated andheats the radiators to create convection air currents.

According to the preferred practice of the invention, the irrigationmethod employed in this combined irrigation and frost protection systemis the so-called drip method which has been used very successfully inarid parts of the world, notably lsreal. The liquid dispensers utilizedin this drip irrigation method are drip dispensers which dispense theirrigation liquid at relatively low volume in droplets at the base ofeach planting. The advantages of this irrigation method are well knownand hence need not be elaborated on in this disclosure. Suffice it tosay that major advantages of the drip method are less liquid evaporationwith the result that water of a higher saline content can be used forirrigation than with other irrigation systems, such as open trench andsprinkler systems, reduced competition between the capillary effects ofthe soil and planting roots to absorb the irrigation liquid, andelimination of water spots and other deposits on the plantings as occurswith the sprinkler-type irrigation systems, particularly when the liquidcontains a fertilizing agent.

The particular combined irrigation and frost protection system describedemploys the drip irrigation method and drip dispensers. The thermalradiators are thermally conductive plates which are mounted on thedispensers in such a way that the plates may be located in the pathsalong which the droplets of irrigation liquid drip from the dispensers.Accordingly, when the combined system is operated in its frostprotection mode, the liquid droplets drip onto and thereby heat theradiators to create convection air currents about the adjacentplantings. In the irrigation mode of the system, the radiators areretracted to permit the irrigation liquid droplets to fall directly ontothe soil. A variety of adjustable drip dispenser configurations for thecombined system are disclosed. The disclosed system also has provisionfor selectively introducing a fertilizing agent into the irrigationliquid.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof a combined irrigation and frost protection system according to theinvention;

FIG. 1A illustrates a modified heating arrangement for the system;

FIGS. 2 and 3 illustrate two different arrangements of the liquiddispensing and thermal radiating units which are placed adjacent thebase of the plantings to be irrigated and/or protected against frostdamage;

FIG. 4 illustrates the manner in which the present system producesconvection air currents about a tree to protect the latter against frostdamage;

FIG. 5 is an enlarged longitudinal section through one of the liquiddispensing and thermal radiating units embodied in the system of FIGS.1-4;

FIG. 6 is an end view of the unit in FIG. 5; and

FIGS. 7-9 are longitudinal sections through modified liquid dispensingand thermal radiating units;

FIGS. 10-12 illustrate three separate embodiments comprising acylindrical tube having a radial hole and a rotatable screw arrangementfor varying the flow rate of liquid through said radial hole.

DESCRIPTION OF THE PREFERRED EMBODIMENT As noted earlier, the presentinvention has two primary aspects, one concerned only with protectingplantings against frost damage and the otherrwith both irrigating theplantings and protecting them against frost damage. The particularinventive embodiment illustrated in the drawings is a combinedirrigation and frost protection system 10 installed in a grove 12 ofplantings 14. In this description, it is assumed that the plantings 14are crop bearing trees, such as fruit or nut trees. While theillustrated system 10 is intended for use as a combined irrigation andfrost protection system for the trees 14, it will become evident as thedescription proceeds that the system may be used in conjunction withsome other tree irrigation system and provide only frost protection.

In general terms, the illustrated system 10 includes at least one liquiddispenser I6 and one thermal radiator I8 (FIG. 4) adjacent the base ofeach tree 14, means 22 for conducting an irrigation liquid such as waterfrom a supply 24 to each dispenser and into heat transfer relation witheach radiator, and means 26 for selectively heating the liquid. Thesystem has two operating modes, namely, an irrigation mode and a frostprotection mode. In the irrigation mode, unheated irrigation liquid issupplied to the dispenser 16 which dispense the liquid to the soil 28 atthe base 20 of the adjacent trees 14. In the frost protection mode, theheating means 26 is operated to heat the irrigation liquid, and theheated liquid is conducted into heat transfer relation with the thermalradiators 18 to heat the latter.

The heated radiators heat the surrounding air by conduction andradiation to produce convection air currents 30 about the adjacenttrees. These convection air currents have the two-fold effect oflowering the minimum air temperature which the trees can withstandwithout permanent damage and circulating the cold air upwardly from andthe warm air downwardly to the base of the trees to raise the airtemperature about the trees.

In this latter regard, it will be recalled that the effect of aircirculation on the minimum air temperature a tree can withstand is knownto exist but not understood, while the effect of air currents on grovetemperature is both known to exist and understood. It will be furtherrecalled that the temperature range between the temperature a tree canwithstand without damage and the temperature at which a tree willsustain permanent frost damage is extremely small and may be on theorder of 4. Accordingly, only a relatively small change in grovetemperature and in the minimum air temperature a tree can withstand maybe sufficient to protect the tree against frost damage.

Referring now in more detail to the illustrated irrigation and frostprotection system 10, the illustrated irrigation liquid dispensers 16are drip dispensers which dispense the irrigation liquid to the trees 14in droplets at relatively low volume. A variety of drip dispensers whichmay be used are illustrated in FIGS. 5-9 and will be described shortly.One or more of the dispensers 16 may be placed adjacent the base of eachtree, depending on its age, size and the amount of water it requires.FIGS. 2 and 3 illustrate two possible arrangements of multipledispensers about each tree.

The irrigation liquid conducting means 22 comprises a piping systemincluding a main supply pipe 32, header pipes 34 extending from thesupply pipe along the tree rows 36, and branch pipes 38 extending fromthe header pipes to the individual dispensers 16 at the base of the tree14. The supply pipe 32 is connected to the irrigation liquid supply 24through a pressure regulator 40, a pipe section 41, a three-way modeselector valve 42 to be referred to presently, a filter 44 and ashut-off valve 46. According to customary practics, a pair of pressuregages 48 are connected to the filter 44 to register the liquid pressuresat the inlet and outlet sides of its filter elements and thereby providea visual indication of the filter efiiciency in terms of the pressuredrop across the filter. An increase in this pressure drop to apredetermined level indicates that the filter elements are dirty andrequire cleaning.

In the irrigation mode of the system 10, the shut-off valve 46 is openedand the selector valve 42 is placed in an irrigation position. In thisposition, the selector valve communicates the outlet of the filter 44directly to the pressure regulator 40 and liquid supply pipe 32.Unheatcd irrigation liquid is then supplied to the drip dispensers I6which dispense the liquid in droplets to the soil 28 at the base oftheir adjacent trees I4. The trees are thereby irrigated in a highlyefficient manner as explained earlier in connection with the existingdrip irrigation systems. The advantages of such a drip irrigation systemwere discussed earlier and need not be repeated here.

The irrigation liquid heating means 26 comprises a heater 50, such as aflash boiler, arranged in parallel with the pipe section 41 between thepressure regulator 40 and selector valve 42. The inlet of this boiler isconnected to the selector valve 42 through a pipe 52. The boiler outletconnects to the pipe section 41 through a pipe 54. Selector valve 42 hasa frost protection position wherein the valve communicates the outlet offilter 44 to the pressure regulator 40 and supply pipe 32 through theflash boiler 50 and its inlet and outlet pipes 52, 54. When the flashboiler is operated in this selector valve position, then the irrigationliquid is heated and the heated liquid is supplied to the dripdispensers 16. As will be explained presently, the thermal radiators 18are mounted below the dispensers in such a way that they can be locatedin the path along which the heated liquid droplets drip from thedispensers. The radiators are thus heated by the irrigation liquid toproduce the convection air currents 30, as explained earlier.

In some cases, it may be desirable or necessary to maintain a relativelylarge supply of heated irrigation liquid in readiness for use. This canbe accomplished by connecting the heater pipes 52, 54 to a liquidreservoir 56 (FIG. 1A) equipped with a recirculating liquid heatingsystem 58 similar to a swimming pool heating systern for maintaining theliquid in the reservoir at a selected elevated temperature. A floatcontrolled valve 60 or the like is connected in the inlet pipe 52 tomaintain the proper liquid level in the reservoir. A pump 62 isconnected in the pipe 54 for pumping heated liquid from the reservoir tothe drip dispenser 16.

FIGS. 5 and 6 illustrate one of the drip dispensers 16. This dispenserhas a body sleeve 64, one end of which is fixed within an end of abranch pipe 38 of the piping system 22. Fitted over and fixed to theother end of the body sleeve 64 is an orifice sleeve 66 containing asmall orifice 68. Within the orifice sleeve 66 is a ball valve 70confronting a beveled valve seat 72 on the end of the body sleeve 64. Ascrew 74 threaded in a sleeve 76 fixed within the outer end of theorifice sleeve 66 is engageable with the ball to limit the maximumclearance between the ball and its valve seat. From this description ofthe drip dispenser 16, it is evident that irrigation liquid supplied tothe dispenser emerges from the latter through the orifice 68 in the formof droplets whose frequency may be regulated by adjusting the screw 74to vary the clearance between the ball 70 and valve seat 72.

The thermal radiator 18 comprises a clip having a thermally conductiveplate 78 with a dished end 80 and resilient clip arms 82 at its oppositeend which snap over the dispenser body sleeve 64. The radiator clip isslidable along the body sleeve 64 between its illustrated operativeposition and an inoperative position against the end of the branch pipe38. In operative position, the dished end 80 of the radiator plate 78 islocated directly below the dispenser orifices 68 so as to be located inthe path along which the irrigation liquid drips from the dispenser.When the liquid is heated in the frost protection mode of the system 10,therefore, the liquid droplets from the dispenser'drip onto and heat theradiator plate 78 to produce convection air currents 30 about theadjacent tree 14. The liquid tends to collect in the dished plate end 80to more effectively heat the plate. In inoperative position, theradiator plate 78 is retracted clear of the liquid droplets from thedispenser 16. The drip dispensers may be mounted at the base of thetrees 14, just above ground level, in any convenient manner.

According to the preferred practice of the invention, the dripdispensers l6 and the pipes of the piping system 22 are constructed of aplastic, such as polyvinyl chloride, having a relatively low coefficientof thermal conductivity to minimize heat loss from the heated irrigationliquid.

FIGS. 7-9 illustrate alternative drip dispensers 16a, 16b, 16c for usein the system 10. Dispenser 166 in FIG. 7 has a needle orifice controlincluding a needle valve 74a threaded in the dispenser orifice sleeve66a and having a valve needle a extending through a conical valve seat720 on a valve seat member 73a within the orifice sleeve 66a. Theradiator plate 78a of the dispenser radiator 18a has a sliding ringattachment 82a to the body sleeve, whereby the radiator may be movedbetween operative and inoperative positions like the radiator 18. Ring82a may have a spike 84a to be inserted into the ground mounting thedispenser in fixed position.

The drip dispenser 16b of FIG. 8 is similar to the dispenser 16a exceptthat the needle valve orifice control of the latter dispenser isreplaced by a simple screw 74b threaded in the body sleeve 64b andhaving a conical head seating against the end of the orifice sleeve 66b.In this case, the threads of the screw form a tortuous passage throughwhich the irrigation liquid flows to the dispenser orifice 68b. Thelength of this passage and hence the rate of the droplets from thedispenser are adjustable by adjusting the screw. in the simplified dripdispenser 16c of FIG. 9, the orifice sleeve is omitted, the orifice 68cis formed directly in the body sleeve 64c, and the orifice control screwMe is threaded in the body sleeve. As in the dispenser 1612, the screwthreads form a tortuous passage for the irrigation liquid. The rate ofthe liquid droplets from the dispenser is regulated by adjusting thescrew.

Referring now to FIGS. 10, l1 and 12 there is illustrated anotherembodiment of the radiator in the form of a thermally conductive metalcylinder 80. The radiator in the preferred embodiment is approximately 2inches long and has a centrally located hole through which the dripdispenser 16 is inserted. In the frost protection mode as shown in FIG.10, the drip opening 68d is located inside the metal cylinder 80 so thathot liquid will fall on the pipe from which the heat will be radiated.FIG. lll illustrates the irrigation mode with the liquid falling on theground directly from the opening 68.

Returning to FIG. 1, the illustrated irrigation and frost protectionsystem 10 has a liquid fertilizer mixer 86 whose inlet is connectedthrough a pipe 88 to the selector valve 42 and outlet is connectedthrough a pipe 90 to the pipe section 41. Selector valve 42 has afertilizer position wherein it communicates the outlet of filter 44 tothe pressure regulator 40 through mixer 86 and its inlet and outletpipes 88, 9'0. In this valve position a liquid fertilizer is mixed withthe irrigation liquid for fertilizing the trees 14.

It is now evident that the illustrated irrigation and frost protectionsystem 10 is operable in irrigation, frost protection, and fertilizingmodes. In its irrigation modes the system dispenses unheated liquid tothe trees 14 through the drip dispensers to irrigate the trees. Thethermal radiators on the dispensers are preferably retracted in thismode so that the liquid droplets drip from the dispensers fall directlyto the soil 28. In the frost protection mode of the system, theradiators are shifted to their frost protection positions and the flashboiler 50 is activated to heat the irrigation liquid. The

produce the convection currents 30 about the trees 14. In thefertilizing mode of the system, the irrigation liquid is directedthrough a fertilizer mixer 86 to effect fertilizing of the trees.

As noted earlier, while the illustrated system is designed for both treeirrigation and frost protection, the irrigation and fertilizing featuresof the system may be eliminated to provide a simple thermal frostprotection system which is operable only in a frost protection mode. Ineither case, the system may be thermostatically controlled toautomatically activate the system in its frost protection mode when theair temperature drops to a preset temperature level. In FIG. 1A, forexample, the pump 62 may be activated by a thermostatic control 92 tosupply heated irrigation liquid to the dispensers 16 when thetemperature drops.

What is claimed as new in support of Letters Patent 1. In a frostprevention system for inducing air circulation about a planting toprotect the planting against frost damage, the combination comprising:

a thermal radiator located above the ground having a substantially largeradiating surface area located adjacent the base of said planting, and

a drip dispenser located above said radiator for dispensing droplets ofheated liquid onto said radiator whereby said radiator is heated by saiddroplets to create convection air currents about said planting.

2. A frost protection system according to claim 1 which includes meansfor heating said liquid at a central location and conducting the heatedliquid in a thermally insulated conduit to said drip dispenser.

3. A frost protection system according to claim 2 including:

thermostatically controlled means for controlling the heated liquid atsaid central location in response to ambient air temperature about theplanting.

4. A frost protection system according to claim I wherein:

said drip dispenser includes means for adjusting the rate at which saidliquid is dispensed from the dispenser.

5. In a combined irrigation and frost protection system which dispensesan irrigation liquid to the soil at the base of a planting in a grove tobe irrigated, the combination comprising:

thermal radiators located above the ground to be placed adjacent thebase of said planting,

a drip dispensing means located adjacent the base of said plantings forconducting said liquid into heat transfer relation with said radiatorsand dispensing the liquid into the soil adjacent the planting, and

means for selectively heating said liquid at a central location andconducting the heated liquid in a thermally insulated conduit to saidthermal radiator,

whereby said irrigation system is selectively operated in an irrigationmode where unheated liquid is dispensed to irrigate said plantings and afrost protection mode wherein said liquid is heated to heat saidradiators and thereby create convection air currents about saidplantings to protect the latter against frost damage. 6. A combinedirrigation and frost protection system according to claim 5 wherein:

each radiator comprises a thermally conductive member, and eachdispenser dispenses said liquid onto its respective radiator. 7. Acombined irrigation and frost protection system according to claim 5wherein:

each dispenser is a drip dispenser which dispenses said liquid indroplets. 8. A combined irrigation and frost protection system accordingto claim 7 including:

means mounting each radiator on its dispenser for adjustment between afrost protection position wherein the radiator is located in the pathalong which the liquid droplets drip from the dispenser and anirrigation position wherein the radiator is spaced from said path. 9. Acombined irrigation and frost protection system according to claim 5wherein:

said dispensing means has an inlet for connection to a supply of saidirrigation liquid, a pair of parallel flow paths between said inlet anddispensers, and a mode selector valve for communicating said inlet tosaid dispensers through either flow path, and said heating meanscomprises a heater in one flow path. 10. The method of protecting aplanting against frost damage, which comprises the steps of:

placing a thermally conductive radiator adjacent the base of saidplanting, and conducting a heated liquid drop by drop into heat transferrelation to said radiator to heat the latter and thereby createconvection air currents about said planting. 11. The method ofirrigating and protecting a planting against frost damage, whichcomprises the steps of: placing a thermally conductive radiator adjacentthe base of said planting, and selectively dispensing an irrigationliquid onto the said adjacent base of said planting to irrigate theplanting in a first mode and heating said liquid and dispensing theheated liquid by drops onto said radiator to heat the latter and therebycreate convection air currents about the planting in a second mode.

1. In a frost prevention system for inducing air circulation about aplanting to protect the planting against frost damage, the combinationcomprising: a thermal radiator located above the ground having asubstantially large radiating surface area located adjacent the base ofsaid planting, and a drip dispenser located above said radiator fordispensing droplets of heated liquid onto said radiator whereby saidradiator is heated by said droplets to create convection air currentsabout said planting.
 2. A frost protection system according to claim 1which includes means for heating said liquid at a central location andconducting the heated liquid in a thermally insulated conduit to saiddrip dispenser.
 3. A frost protection system according to claim 2including: thermostatically controlled means for controlling the heatedliquid at said central location in response to ambient air temperatureabout the planting.
 4. A frost protection system according to claim 1wherein: said drip dispenser includes means for adjusting the rate atwhich said liquid is dispensed from the dispenser.
 5. In a combinedirrigation and frost protection system which dispenses an irrigationliquid to the soil at the base of a planting in a grove to be irrigated,the combination comprising: thermal radiators located above the groundto be placed adjacent the base of said planting, a drip dispensing meanslocated adjacent the base of said plantings for conducting said liquidinto heat transfer relation with said radiators and dispensing theliquid into the soil adjacent the planting, and means for selectivelyheating said liquid at a central location and conducting the heatedliquid in a thermally insulated conduit to said thermal radiator,whereby said irrigation system is selectively operated in an irrigationmode where unheated liquid is dispensed to irrigate said plantings and afrost protection mode wherein said liquid is heated to heat saidradiators and thereby create convection air currents about saidplantings to protect the latter against frost damage.
 6. A combinedirrigation and frost protection system according to claim 5 wherein:each radiator comprises a thermally conductive member, and eachdispenser dispenses said liquid onto its respective radiator.
 7. Acombined irrigation and frost protection system according to claim 5wherein: each dispenser is a drip dispenser which dispenses said liquidin droplets.
 8. A combined irrigation and frost protection systemaccording to claim 7 including: means mounting each radiator on itsdispenser for adjustment between a frost protection position wherein theradiator is located in the path along which the liquid droplets dripfrom the dispenser and an irrigation position wherein the radiator isspaced from said path.
 9. A combined irrigation and frost protectionsystem according to claim 5 wherein: said dispensing means has an inletfor connection to a supply of said irrigation liquid, a pair of parallelflow paths between said inlet and dispensers, and a mode selector valvefor communicating said inlet to said dispensers through either flowpath, and said heating means comprises a heater in one flow path. 10.The method of protecting a planting against frost damage, whichcomprises the steps of: placing a thermally conductive radiator adjacentthe base of said planting, and conducting a heated liquid drop by dropinto heat transfer relation to said radiator to heat the latter andthereby create convection air currents about said planting.
 11. Themethod of irrigating and protecting a planting against frost damage,which comprises the steps of: placing a thermally conductive radiatoradjacent the base of said planting, and selectively dispensing anirrigation liquid onto the said adjacent base of said planting toirrigate the planting in a first mode and heating said liquid anddispensing the heated liquid by drops onto said radiator to heat thelatter and thEreby create convection air currents about the planting ina second mode.